Traditional telephone systems include a common controller, sometimes referred to as a call controller, which is responsive to service requests signalled from associated telephone sets for causing a switching apparatus or network to provide telephone communication paths between calling and called telephone sets. More than twenty years ago a PBX was first marketed by the Northern Electric Company Limited with the trademark SL 1. In this PBX the call controller is provided by a digital processor with associated memory. The memory includes stored data for defining terminal locations of telephone sets, and instruction sets by which the digital processor is operative for generating connection instructions. Connection memories in a time division multiplex (TDM) switching network contain the connection instructions generated by the call controller for directing transfers of pulse code modulated (PCM) information bits between frame organized synchronous channels to provide communications paths between telephone sets. Each telephone set is served by a line circuit which couples information between a channel in the switching network and the telephone set.
Signalling between the telephone sets and the call controller is provided for with peripheral circuitry which collects signalling information in a channel reserved for this purpose from the line circuits as it occurs and forwards it to the call controller. Conversely signalling destined for individual telephone sets is distributed to the respective line circuits via the same channel.
Control and status of the line circuits and of the switching network is exercised by the call controller via a control bus which includes address, data, control, and timing leads.
A more complete review of the first marketed SL1 PBX is available with reference to articles published by Bell Northern Research Limited in Telesis in February of 1975.
The essence of the typical PCM TDM telephone exchange is embodied in its telephony software which provides for basic telephone call operations as well as the exercise of a multitude of telephony features and conveniences. One of the distinguishing features of the telephony software is that of having a plurality of hierarchically organized priority interrupts which permits the call controller to shelve a lower priority task when a higher priority task comes along. Judicial assignments of the priorities of tasks permits the PBX to respond apparently instantaneously to user requests for service and telephony features. Hence the call controller is always occupied by the highest priority task that is available at the time. This is in contrast to the typical general purpose computer which is batch processing oriented to complete one task at a time.
The evolution of hardware and software technology and economies of scale have reduced physical size while increasing functionality to such an extent that computers have in the last 10 years become personal and commonplace on the users desk top. PBXs have experienced a similar evolution now requiring less than a cubic meter of space where 20 years ago a large equipment room was needed to house the same or less functionality.
During this evolution the development of telephony features such as automatic call distribution, voice messaging, networking and automated attendant to mention but a few, has brought about the addition of feature servers to the basic PBX. The feature sever permits the execution of feature rich telephony services without unduly burdening the call controller or otherwise detracting from the basic telephony functions of call set ups and take downs. In many examples the features server is based upon the traditional general purpose computer which is batch processing oriented to complete one task at a time. As the personal computer (PC) has evolved with faster and faster machine speeds it has been realized that telephone system functionality can be incorporated into a PC through the application of switching emulation software and the addition of hardware interface circuits for connecting a few lines and trunks with an internal PC communications bus. Various of the PC's resources can be put to use in providing telephony features that would typically have required a features server. In very small systems a significant cost advantage may be realized as the PC may still be used for other traditional PC tasks while in addition screen based telephony features can also be implemented via other user's PCs coupled through a local area network (LAN). Author Harry Newton, in an article titled "The Telephony Server and The Communicating PC Client", published in the April 1995 issue of TELECONNECT speculates on the various communications services that will become available via the PC and mentions several product offerings.
One of the current problems with using a PC for switching telecommunications is that neither the existing telecommunications software nor the principles upon which it is based are directly applicable within the PC architecture. The existing telecommunications software has been developed over the past more than 20 years for switching facility call controller architectures that have been specialized toward the telecommunications task with the assumption that call control is provided virtually instantaneously in apparent real time. One of the primary restraints to providing telecommunications switching in a PC is the absence of fully featured robust telecommunications software for the PC architecture.
This problem is somewhat overcome in a very recent example of the combination of a PBX call controller within a personal computer. This has an advantage that the PBX call controller is able to rely upon power and bulk memory resources of the PC. Furthermore PBX features such as voice mail and automated attendant, usually provided by a telecommunications features server, can be provided by the physical resources of the PC. Although the processor in the personal computer and the call controller of the PBX have dissimilar and non compatible input/output (I/O) bus structures, each is able to communicate with the other and use common bulk memory resources via a standard personal computer industry (PCI) local bus. Each element communicates with the PCI bus via a PCI bus controller or interface. Examples of integrated circuits for this purpose are identified in the code series S5930-S5933 and are available from Applied Micro Circuits Corporation at 6195 Lusk Blvd. in San Diego, Calif. 92121. In this example successful transmission of telephony signals of synchronous origin via the PCI bus relies upon there being sufficient PCI bus bandwidth to accommodate all of the demands for such telephony transmissions as well as the requirements for normal PC operations. If the telephony traffic is sufficiently heavy to substantially dominate the PCI bus there is no practical advantage to the combination of the PBX and the PC.